Means for measuring the root mean square value of a complex electrical wave



L W. SEPMEYER.

Aug. 2, 1960 2,947,935

MEANS FOR MEASURING THE ROOT MEAN SQUARE VALUE OF A COMPLEX ELECTRICALWAVE Original Filed Aug. 21, 1952 46 g2 REc'oQDi? 5/45M/6 l/vPllr INPUT64 6 6 BI/QISIIA IG 8/ j N U cz-z W emqm g7 82 52 INVENTOR.

ATTORNEYS between the components. acoustic measurements, the R.M.S.detecting element must have a time constant less than one-tenth second;a. time constant of ten milliseconds or less is preferred for recordingdata on a fast graphic recorder of-the power MEANS FOR MEASURING THEROOT MEAN SQUARE VALUE OF A COMPLEX ELECTRI- CAL WAVE Ludwig W.Sepmeyer, 1862 Comstock Ave.,

, Los Angeles, Calif.

Continuation of application Ser. No. 305,717, Aug. 21,

v 1952. .This application Sept. '5, 1956, Ser. No. 608,654

1 Claim. (Cl. 323-151 v (Granted under Title 35, US. Code (1952), sec.266) The invention described herein may be manufactured and used by orfor the Government of the United States of America for governmentalpurposes without the payment of any royalties thereon or therefor.

This invention, a continuation of US. patent application Serial No.305,717, filed August 21, 1952, now "abandoned, relates to a measuringinstrument of the typewherein the temperature coeflicient of resistanceof a thermal element in an electrical bridge is changed in a welldefined manner by the variable to be measured. 7

,It is desirable in several fields of measurement to be able to measurethe root mean squarethereinafter abbreviated as R.M.S.) value of acomplex electrical wave. This is especially true in acoustic noisemeasurements.

"Most of the available instruments for measuring waves are'of theaverage reading and peak reading types. ;On

to the signal output lead 18 at point .20.. The second arm of the firstside of said bridge includes fixed resistors 22 and 24 and. thermalelements 26 and 28 connected as shown to point 20 and to point 30. Asecond biasing input lead 32 is also connected to point 30. Thesecondcomplex waves these available instruments give readings 'which'aredependent not only npon the number of.com-

ponents in the wave, but-also on the-relative phase shift In orderto beuseful for level type. ThemOst familiar known type of R.M.S. type ofinstrument is thethermocouple which is characterized thereof which callsfor a very sensitive DEL-amplifier and itsaconcomitant ditficulties. Thesameobjections of '40 by. slow response due to thermal inertia of theheater 1 and low safety .margin to overload. Another disadvan- "tage ofthe thermocouple is theextre'mely low output slow response and lowoutputalso applyto devices makjug use of electrostaticand'electromagnetic attraction; vacuum tubes which exhibit square-lawtransfer characteristics have a very limited peak-to-R.M.S. I capability"and the useful-characteristics are availableonly at a critical gridbias and screen voltage. In the instant invention, the detecting thermalelement comprises alength of small diameter wire as a resistance elementwhich is arranged in a suitable circuit to provide a highly sensitiveR.M.S. meter.

An object of the invention is to provide an improved apparatus whereinthe characteristics of a resistive impedance are utilized for indicatingthe R.M.S. value of a complex wave over a wide frequency range.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same becomes better understood byreference to the following description.

Fig. 1 is a schematic diagram showing the thermal element in relation tothe circuit components which it is used; and

Fig. 2 is a similar diagram. of a modified form of the invention. Y

The circuit shown in Fig. 1 comprises a Wheatstone bridge having as onearm a fixed resistor 12 which is connected to the biasing input lead 14at .point 16 and side of the bridge is.identical with the first sideexcept for the addition .of signal input leadsv34 and 36 connectedacross the series-parallel arrangement of fixed resistors 38'and 40withzthermal elements 42 .and- 44.

Fixed. resistor 46 is'connected intothe bridge at point 16-and atpoint48 to which the second signal .output lead 50 is connected. Thermalelements 26, 28, 42 and 44in this instance may be .short lengths of veryfine wire. .A 35 micro-inch diameter platinum Wollaston wire gives verygood results. .The .response time of such an element in air is abouttonemillisecond; this time may be shortened by placing the wire in anatmosphere of hydrogen or helium. The thermal elements must be se---lected for matching characteristics both as to the total --resistanceand as to .the'change in resistance with increase in temperature. A.sixmillimeter length of one micron wire will have a nominal resistance ofabout 2,000 ohms for one element. With ,a 2.5 milliamperes current flowthrough the elements provided through biasing input leads 14 and 32, amaximum signal output voltage of slightly over one volt may be attainedfrom a circuit using such elements. The direct current passing throughthe thermal elements behaves as a bias to main-- tain the elements at asuitable operating value; this bias is selected to be-a-bout one-half ofthe total current which the thermal elements will safely carry. The lowthermal inertia of the meter is attributablelto the very small diameterof the thermal element wires.

In the operation of the circuit thus far described, a steady D.C.biasing input voltage is applied to leads 14 and 32. Assuming no signalinput voltage is applied to leads 34 and'36, the bridge will be balancedand no potential difference will exist between points 20 and 48. Thebridge will remain in balance even though the ambient temperature maychange since the resistance of the two similar arms of the bridge willvary in the same manner. the right i and side of the bridge is tocorrect for ambient temperature variations; it is evident that thecompensating circuit comprising impedances 22, 24, 26 and 28 may bereplaced by a single fixed resistor if the am- "bient temperaturechanges are-smalls If an AC. signal input voltage is applied to leads 34and 36, the distribution of current through the bridge will be changedbecause the temperature of thermal elements 42 and 44 will increase andtheir resistance will increase accordingly. This increase in resistancein one side of the bridge creates an unbalance and a potentialdifterence will result between points 20 and 48. The output of leads '18and 50 may be applied to external circuitry such as any of the variousknovm types of indicating or recording means, for example a fast graphicrecorder 52. Since the ratio of the resistance of resistor 38 to that ofresistor 40 is the same as the ratio of the resistance of thermalelement 42 to that of thermal element 44, the internal bridge comprisingthese impedances is balanced with respect to the input signal and therecan be no potential difierence between points 30' The sole purpose ofthe thermal elen-lentsin well adapted to balancedto-ground amplifiersshown other part goes through resistor 80, point 81, and thermal element82 before reaching point'78. The A.C. signal input leads 84 and86:areconncted to points 81 and 75 respectively. Potentiometer winding 88is connected across signal input leads 84 and 86; potentiometer slider'90 isconnected to lead 92 which, with lead '94, provides means forconnecting the signal output of the arrangement to an external unit suchas, for example, a fast graphic recorder 52. as in the Fig. lembodiment. The resistance of impedances-65, 7.0, 74 and 80 is madelarge in this embodiment in comparison to the resistance of thermalelements 76 and 82 so that the elements will "operate underconstant.D.C. bias conditions and also so that little A.C. power islostin the elements. This difference is not essential for properoperation of the apparatus. Resistor 70 may be replaced by a thermalelement to provide complete ambient temperature compensation for theapparatus.

In the operation of the above described apparatus, an A.C. signal inputvoltage: is applied to leads 84 and 86, and a biasing D;C. input voltageis connected to leads 60 and 62. Rheostat 65 isadjusted so that the sameD.C. potential exists at point 68 and slider 90. The location of slider90 has substantially no eflect on this adjustment. Potentiometer slider90 is then adjusted so thatvno part of the input A.C. signal can bedetected between point 68 and slider 90. This feature of adjustabilitymakes it possible to compensate for small diiferences which existbetween thermal elements. It has been found that the A.C. balance holdsgood over long periods of time.

Since the thermal elements as Well as the other circuit components areresistive 'impedances, high frequencies may be handled. The upper limit,which is in the microwave range, is limited by such phenomena as skineffects and standing waves, and there is no lower frequency limit sincethe apparatus operates perfectly on D.C. input signals. Although shortthermal time constants may not be required for many applications, it ishighly desirable to be able to select any time constant over a widerange which can :be done bymeansiof well known integrating circuitry.The output of the R;M.S. meter may be fed into other units, .such as adifferential amplifier of the DC. type or of the chopper amplifier type.The output 2,947,935 v a i Other elements characterized by electricalresistance values which are dependent upon temperature or appliedcurrent or voltage, including non-metallic thermal elements such as beadthermistors, may be used in the described apparatus. In general,however, they are present- 1y judged to be less effective. In the caseof the bead thermistor, for example, the time constant is longer thanwhen wire is used because the resistance material-is on a base whichtakes an appreciable time to heat up. The fact that certain suchelements may have a negative temperature cocificient of resistance is ofno consequence in this application. It should also be noted that theapparatus shown in Figs. 1 and 2 will operate on an A.C. biasing inputsupplied to loads 14, 32 in Fig. 1, and to leads 60, 62 in Fig. 2,correspondingly providing A.C. carrier outputs, but D.C. biasing inputis preferred.

Obviously many modifications. and variations of the present inventionare possible in the light of the above teachings. It is therefore to'beunderstood that within the scope of the appended claim the invention maybe practiced otherwise than as specifically described.

What is claimed is:

An electrical apparatus'for deriving the time-variable root-mean-squarevalues of a complex input voltage, comprising: an electrical networkhaving four bridge-connected arms, two-adjoining arms of said networkhaving thermally-responsive elements each having an electricalresistance which is dependent upon the temperature of said elements inresponse to current therethrough, and said elements each having athermal time costant which is short relative to the periods in whichvariations in said root-mean-square values occur, the other twoadjoining arms having like resistance, means for applying'abridgeenergizing voltage between the junction of said two adjoining armshaving thermally-responsive elements and the junction of said other-twoadjoining arms, first resistive means connected between said junctionsand having an output terminal connected to a variable intermediate pointthereof, second resistive means connected between the remaining twojunctions of said bridge-connected arms and having an'output terminalconnected to a variable intermediate point thereof, and means forapplying said complex input voltage between said remaining twojunctions, whereby said electrical apparatus provides at said outputterminals a time-variable output voltage corresponding to and serving asa measure of the timeof the meter may also be utilized in a servo systemto maintain a signal at a constant specified level.

variable root-mean-square values of said complex input voltage.

ReterencesCited in the file of this patent UNITED STATES PATENTS2,459,104 Gilbert Ian. 11, 1949 2,487,697 Conviser Nov. 8, 19492,525,179 *Polye Oct. 10, 1956

